[FIX] raise error for empty text

[rafaelleaygalenq]

Description

Bugfix for issue #205 - Raise error when empty string is passed into `remove_stopwords`

Related Issue

https://github.com/artefactory/NLPretext/issues/205

Type of Change

• [ ] 📚 Examples / docs / tutorials / dependencies update
• [x] 🔧 Bug fix (non-breaking change which fixes an issue)
• [ ] 🥂 Improvement (non-breaking change which improves an existing feature)
• [ ] 🚀 New feature (non-breaking change which adds functionality)
• [ ] 💥 Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🔐 Security fix

Checklist

• [x] I've read the CODE_OF_CONDUCT.md document.
• [x] I've read the CONTRIBUTING.md guide.
• [x] I've updated the code style using make codestyle.
• [x] I've written tests for all new methods and classes that I created.
• [x] I've written the docstring in Google format for all the methods and classes that I used.